Exploring the structural, morphological, and dielectric properties of Sr and La single-doped and co-doped calcium copper titanate

Solid solutions comprising CaCu₃Ti₄O₁₂ (CCTO), Ca_0.94La_0.04Cu₃Ti₄O₁₂ (CLCTO-4), Ca_0.88La_0.08Cu₃Ti₄O₁₂ (CLCTO-8), Ca_0.9Sr_0.1Cu₃Ti₄O₁₂ (CSCTO)_, and Ca_0.785Sr_0.2La_0.01Cu₃Ti₄O₁₂ (CSCLTO) were prepared by an environmentally friendly and modified solid-state sintering method to study the crystal structure, surface morphology, dielectric response, and optical properties of the synthesized materials. The XRD patterns ensure the synthesis of a pure phase CCTO with minimal impurities of CuO and TiO₂ found in CCTO, CLCTO-4, and CSCTO samples. The inclusion of both Sr and La in CCTO results in a reduction in the average grain size and enhanced uniformity, along with an enlargement in lattice spacing from 0.260 nm to 0.261 nm for the 220 net plane for CLCTO-8, as evidenced by SEM and TEM images. The impedance spectroscopy depicts co-doping with Sr and La results in a simultaneous increase in dielectric constant (k) and reduction in dielectric loss (tanδ). The CLCTO-8 ceramic exhibits the highest dielectric constant (40,372 at 40 Hz) and lowest dielectric loss (0.007 at 41.5 kHz). The occurrence of mixed Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺ valence states in the co-doped material lattices fosters dipole polarization, increasing the ceramic's dielectric constant. The magnitude of the band gap, as determined by the Tauc plot, shows that the optical band gap increases with La% but drops with Sr, possibly because of the electronic transition between valence and conduction bands.